1. Technical Field of the Invention
The present invention relates generally to a dry safety device for an inflammable gas, and more particularly to a dry safety device for shutting off a gas passage to prevent adverse effects to the supply source of inflammable gas when a dangerous condition such as back fire or fire occurs during a gas welding operation or a gas cutting operation.
2. Description of Related Art
As a safety measure in gas welding or gas cutting operations using an inflammable gas, it is known to provide a dry gas safety device having a gas passage shutoff mechanism to prevent a counter flow or backflow of the inflammable gas accompanied with back fire that may occur in a hose connecting a burner torch to a bomb filled with the inflammable gas. Such a dry safety device is constructed so that when a counter flow of gas occurs from a secondary side toward a primary side, the device is automatically operated by the counter-flow pressure to shut off the gas passage, and when the counter-flow phenomenon ends or disappears, the shutoff mechanism can be manually reset and used again. However, this safety device is not constructed to prevent the counter flow of gas in situations when a counter-flow pressure is not produced. One example of such a situation is in normal use conditions, when a valve of the burner torch is closed, and the inflammable gas remains in the hose. In such a state, if a serious situation occurs in the surrounding environment and the ambient temperature is raised too high, or if a fire happens, the hose may be broken and fire may enter the hose, creating a very dangerous condition. Namely, in a conventional dry safety device which is operated by the counter-flow pressure produced by back fire, in the event the hose is broken and the gas escaping from the broken hose simply burns, a counter-flow pressure is not generated and in such a state, this dry safety device does not function.
In order to overcome this drawback, JP-A-2001-289346 proposes a dry safety device for an inflammable gas comprising a passage shutoff mechanism to shut off the gas passage by use of the backflow pressure, which further comprises a high-temperature shutoff mechanism capable of shutting off the gas passage in which the inflammable gas flows even when the ambient temperature is raised high. This high-temperature shutoff mechanism has a valve seat located at a secondary side open port from which the inflammable gas exits the dry safety device and flows to the burner torch, and a shutoff valve temporarily fixed by a low-temperature solder in a retracted position, opposing the valve seat. When a fire occurs and the ambient temperature rises, the low-temperature solder is melted, and the temporarily fixed shutoff valve is released and moved by a spring to an extended position where the extended shutoff valve presses against the valve seat to close the gas passage.
Usually, at the outer periphery of the shutoff valve, an elastic packing such as O-ring is provided so that when the shutoff valve is pressed against the valve seat, the elastic packing will closely fit on the valve seat in a gas-tight manner to securely shut off the gas passage. However, since this shutoff valve is disposed at the secondary side open port, when a counter flow accompanied by back fire occurs, flames that reach the shutoff valve pass the periphery of the elastic packing and damage the packing. As mentioned above, the known dry safety device is constructed so that when the counter-flow phenomenon from the secondary side toward the primary side disappears, the passage shutoff mechanism is reset and used again. Accordingly, whenever the counter-flow phenomenon accompanied by back fire occurs, the elastic packing of the shutoff valve is further deteriorated. As a result, when a fire occurs near the shutoff valve without generating a counter-flow pressure, even if the shutoff valve of the high temperature shutoff mechanism moves to the extended position, the gas passage may sometimes not be securely closed.